Electron beam evaporation sources have been used as a convenient means in recent years for evaporating high vapor temperature materials such as uranium and other refractive metal elements. Such a technique has been found to be useful for the evaporation of uranium in high quantities for production level isotope separation as is discussed in U.S. Pat. No. 3,772,519 and U.S. Pat. application Ser. No. 328,954, filed Feb. 2, 1973. As used in such apparatus, it is preferable to employ a linear electron beam evaporation source as disclosed in the above-referenced application to provide a long line source of uranium vapor for subsequent isotopically selective ionization and separation. Such a vapor source is also preferably operated at a high vaporization rate for increased enrichment efficiency.
The vapor generated by such a technique, particularly in the high densities employed for isotope separation, has produced therein vapor particles in the ionized state but with a generally low thermal energy. For efficient isotope separation, the vapor source is generally free of obstructions directly above and to both sides of the line source of vapor to permit full utilization of the vapor generated. Accordingly, some of the ions produced in the vapor will easily find their way into all portions of the vaporization chamber including the region of high accelerating voltage for the electron beam used to create vaporization. Once the wandering ions have reached this potential region, they are strongly accelerated by the several tens of Kilovolts of accelerating potential toward the filament or cathode where they impact producing a rapid filament degradation from sputter ejecting of filament particles. Additionally, the heat build-up in the filament due to the ion current striking it will further accelerate filament wear.